An organic EL device is a spontaneous light emitting device which utilizes a phenomenon that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang et al. of Eastman Kodak Company [C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Page 913, 1987], many studies have been conducted on organic EL devices using organic materials as the constituting materials. Tang et al. used tris(8-quinolinolato)aluminum for the light emitting layer and a triphenyldiamine derivative for the hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excitons which are formed by blocking and recombining electrons injected from the cathode can be increased, and that the excitons formed in the light emitting layer can be confined. As the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting and light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer are well known. In order to increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
Usually, driving or storing the organic EL devices under an environment of elevated temperatures causes adverse influences such as changes of luminescent colors, degradation of current efficiency of light emission, increase of their driving voltage, reduction of lifetime in their light emission, etc. In order for preventing the adverse influences, it was necessary to elevate a glass transition temperature (Tg) of a hole transporting material. Accordingly, it is necessary for the hole transporting materials to have many aromatic groups in molecules thereof (for example, aromatic diamine derivatives disclosed in Patent Document 1, aromatic fused ring diamine derivatives disclosed in Patent Document 2), and usually, structures having 8 to 12 benzene rings are preferably employed.
However, in a compound having a symmetrical structure in a molecule, crystallization is liable to occur upon production of the organic EL device through the formation of a thin film by using those hole transporting materials. As a result, there arises a problem such as clogging of an outlet of a crucible to be used in vapor deposition or a reduction in yields of the organic EL device due to generation of defects of the thin film resulting from the crystallization. In addition, a compound having a large number of aromatic groups in any one of molecules thereof generally has a high glass transition temperature (Tg), but has a high sublimation temperature. Accordingly, there arises a problem in that the lifetime is short, because a phenomenon such as decomposition at the time of the vapor deposition or the formation of a nonuniform deposition film is expected to occur.
Further, although Patent Documents 3 to 5 report about compounds having dibenzofuran structures, they disclose structures having dibenzofuran as a central skeleton of a diamine compound. Patent Documents 6 to 8 disclose about compounds having dibenzofuran structures through an aryl group in monoamines thereof, however, the compounds do not sufficiently improve the performance as the organic EL device.
Also, Patent Documents 9 to 11 report about monoamine compounds having a terphenyl group, however, the compounds are only used for electrophotographic photoreceptors. Although Patent Documents 12 to 14 disclose about usages as materials for the organic EL device, the materials do not sufficiently improve the performance as the organic EL device.
As mentioned above, there are many reports about the organic EL devices with prolonged lifetime. However, the lifetime is still insufficient. Accordingly, development of an organic EL device having a more superior performance has been eagerly desired.
Patent Document 1: U.S. Pat. No. 4,720,432
Patent Document 2: U.S. Pat. No. 5,061,569
Patent Document 3: JP 2005-112765A
Patent Document 4: JP 11-111460A
Patent Document 5: WO 2006/122630
Patent Document 6: WO 2006/128800
Patent Document 7: JP 2006-151844A
Patent Document 8: JP 2008-021687A
Patent Document 9: JP 2-134643A
Patent Document 10: JP 2-190862A
Patent Document 11: JP 3-118548A
Patent Document 12: JP 6-228062A
Patent Document 13: JP 2001-196183A
Patent Document 14: JP 2006-352088A